1. Field of the Invention
The present invention relates to a zoom lens system, in particular, to a zoom lens system for a medium-format camera in which the diagonal length (2y) of the format is more than 60 mm, and having a finder optical system and a photographing optical system being separately provided.
2. Description of the Related Art
In a zoom lens system for a compact camera in which a finder optical system and a photographing optical system are separately provided, there is no need to have a long back focal distance unlike in a zoom lens system for a single lens reflex (SLR) camera. Accordingly, a compact camera generally employs a telephoto type lens system in which positive and negative lens groups are provided in this order from the object side, while a retrofocus type lens system is generally employed in a SLR camera.
In a telephoto type two-lens-group zoom lens system, distribution of power over the front and the rear lens groups is an important factor to reduce aberrations and to make a camera compact. Furthermore, in order to reduce aberration fluctuations upon zooming, correction of aberrations at each lens group is required. However, it has been difficult to correct aberrations in a zoom lens system in which the half angle-of-view at the short focal length extremity is about 35.degree., and the zoom ratio is about 1.7, and the F-number at the long focal length extremity is 6.7 or less.